1. Field of the Invention
This invention relates to processes for providing substantially planar surfaces at selected points in silicon semiconductor device processes.
2. Related Art
Semiconductor devices formed in silicon substrates, such as bipolar transistors or metal-oxide semiconductor (MOS) transistors, typically employ multiple layers of metal (or other high conductivity material) over device features (such as a MOS gate) which protrude from the substrate's principal surface. The metal layers may or may not be separated by dielectric layers.
These overlying metal and dielectric layers conform, approximately, to the surface formed by the principal surface of the substrate and the protruding device features. As layers are added, irregularities in the upper surface of the immediate underlying layer due to the protruding features, become more and more exaggerated. In extreme cases, an overlying layer will become discontinuous at a site where a series of layers have been stacked over a recess. More commonly, the exaggerated irregularity will leave the upper layers with numerous fragile stress points.
One early attempt to ameliorate the negative effects of underlying irregularities on multiple overlayers, was to deposit borophosphosilicate glass (BSPG) as an overlayer, and reflow the glass. This provided a smoothly contoured surface over device features protruding from the substrate surface, but the smoothly contoured surface still roughly conformed to the underlying features and was thus nonplanar. This nonplanar surface would eventually result in the above described layer discontinuities or stress points if multiple layers were formed over it.
Further, as device feature sizes are reduced to 0.25 .mu.m and smaller, what had been acceptable minute faults in overlayers become intolerable, process limiting barriers.
Although use of a BPSG reflow layer was an improvement, a method of providing a planar surface over semiconductor device features which protrude from the principal surface of a silicon substrate is highly desirable and, heretofore, unavailable.